This invention relates to a novel method for synthesizing borazine. Borazine has been recognized as a useful precursor to the ceramic material boron nitride. Although boron nitride is easy to obtain in powder form from the pyrolysis of simple, inexpensive reagents, it is not feasible to convert boron nitride powders into forms such as fibers and coatings. The need for more processible boron nitride has encouraged research groups to seek new ways to obtain these forms. Polymers that contain boron and nitrogen, such as polymers formed from the monomer borazine, offer an attractive approach. See, for example, Paine, R. T. and Sheddon, L. G., "Borazine-based Polymers Close in on Commercial Performance," Chemtech 1994, 29-37. Unfortunately, practical applications of borazine continue to be held back by the absence of an efficient, economical synthetic route.
Borazine was first prepared by Stock (Chem. Ber. 1926, 59B, 2215-2223) by the thermal decomposition of the diammoniate of diborane. Borazine has also been observed as a product in several other reactions (e.g., Stock, Chem. Ber. 1929, 62B, 90-99; Stock, Chem. Ber. 1930, 63B, 2927-2937; Stock, Chem Ber. 1932, 65B, 1711-1724; Schlesinger, J. Am. Chem. Soc. 1936, 58, 409-414; Schlesinger, J. Am. Chem. Soc. 1938, 60, 1296-1300; Schlesinger, J. Am. Chem. Soc. 1938, 60, 2297-2300; Wilberg, Chem. Ber. 1940, 73B, 209-232; Wilberg, Naturwissenschaften 1948, 35, 182-188; Videla, Proc. Int. Conf. on Peaceful Uses of Atomic Energy, 1955, 8, 619; Emeleus, J. Chem. Soc., 1959, 1306-1307; Burg, Inorg. Chem. 1973, 12, 1448-1450); however, none of these are convenient for laboratory preparations. One of the best laboratory syntheses of borazine has involved the preparation of B-trichloroborazine, (Eq. 1) and its subsequent reduction by metal borohydrides. (Eq. 2. ) EQU 3 BCl.sub.3 +NH.sub.3 .fwdarw.2,4,6-Cl.sub.3 B.sub.3 N.sub.3 H.sub.3 +6HCl(1) EQU 2,4,6-Cl.sub.3 B.sub.3 N.sub.3 H.sub.3 +3 NaBH.sub.4 .fwdarw.H.sub.3 B.sub.3 N.sub.3 H.sub.3 +3 NaCl+3/2 B.sub.2 H.sub.6 ( 2)
Even with the reported improvements (e.g., Zaharkin, Metallorg. Khim, 1993, 6, 381-2), this approach has a number of limitations including relatively small scales, long reaction times, difficult purification, the use of potentially carcinogenic solvents (i.e. chlorobenzene), and requiring the handling of air-sensitive materials, including B-trichloroborazine and the diborane that is generated during the reduction step.
Two larger scale methods for the preparation of borazine have been achieved, but these methods employ equipment or procedures not generally feasible in the laboratory. A commercial procedure is described in U.S. Pat. No. 4,150,097 to Hough et al. In this process, the batch pyrolysis of ammonia-borane dissolved in diglyme was found to produce borazine in 69-71% yields on 0.53 g scales. A more elaborate continuous flow process, involving a heated vertical steel reactor, was then used to make larger quantities of borazine, but the complicated design would be difficult to engineer in the laboratory.
A second procedure is based on the original discovery by Schlesinger (J. Am. Chem. Soc. 1951, 73, 1612-1614) that the high temperature (300.degree. C.) solid-state reaction of lithium borohydride and ammonium chloride yields borazine. Mikheeva and Markina (J. Inorg. Chem., USSR 1956, 56-63) optimized these reactions and found that yields of 38-41% were obtained with a 2:1 excess of ammonium chloride, but the reaction required constant agitation by an unusual "motorless shaker" Similarly, Volkov (J. Inorg. Chem. 1970, 15, 1510-1513) optimized the sodium borohydride and ammonium chloride reaction at 230.degree. C. to produce borazine in yields of 13-23%, but again, an unusual apparatus consisting of a steel reactor and shaker and a metallic nickel trap was required. These solid-state reactions employ inexpensive starting materials, but have the disadvantages of complicated apparatus, variable and low yields and the formation of side products, such as chloroborazine and diborane, which complicate purification and handling.
Haworth (Chem. Ind. (London) 1960, 559-560) has reported that the reaction of sodium borohydride and ammonium chloride in etheral solvents at 190.degree.-230.degree. C. gives borazine in 35% yields. When the present inventors repeated this reaction in tetraglyme solution, however, significant amounts of chloroborazine, ammonia, and acetylene were found, necessitating careful, repeated fractionations of the products to give variable yields of 3-40%. There therefore remains a need for a high yield, economical and convenient method for synthesizing borazine.